As a vehicular headlamp, there is known a headlamp (ADB: Adaptive Driving Beam) for controlling lighting of a travelling light (driving beam) of a headlamp of a vehicle in accordance with a condition of an oncoming or preceding vehicle in front of the vehicle.
Such a vehicular headlamp controls the light distribution state of the headlamp of the vehicle so that, when another vehicle is present in front of the vehicle, for example, the illumination is cut off only in a portion where the other vehicle is located. Such a vehicular headlamp is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei. 07-108873.
Such a vehicular headlamp of the conventional example described above is provided with a camera at a predetermined position in the front part of the vehicle (for example, at the center upper part of the front window), and the vehicle body of a target vehicle (preceding vehicle or oncoming vehicle) imaged by the camera, or the position of the tail lamp or headlamp thereof is detected by image processing. Then, light distribution control is performed so that the detected portion of the preceding vehicle or the oncoming vehicle is not irradiated with light from the traveling light of the vehicle.
Japanese Patent Application Laid-Open No. Hei. 06-191346 discloses the use of a liquid crystal element for controlling the light distribution state of a headlamp.
FIG. 1 is an automobile headlamp system disclosed in Japanese Patent Application Laid-Open No. Hei. 06-191346. Reference numeral 80 denotes a light source, 81 denotes a projection unit, 82 denotes a reflecting mirror constituting the projection unit 81, 83 denotes a collimator lens constituting the same projection unit 81, 84 denotes an optical shutter that forms an optical image, 85 denotes an ultraviolet cut filter, 86 denotes an oncoming vehicle detection sensor, 87 denotes an optical shutter drive circuit, 88 denotes a light distribution control circuit, 89 denotes an attitude change detection sensor, 90 denotes a speed sensor, and 91 denotes a steering rotation angle detection sensor. In this conventional art example, liquid crystal is used as a light transmittance variable element for forming an optical image, which is provided in the optical shutter 84.
In the automobile headlamp system including the above-mentioned components, the relationship between these components and the operation thereof will be described.
Light rays emitted from the light source 80 are converted into substantially parallel light rays by the reflecting mirror 82, and are irradiated onto the optical shutter 84. Then, an optical image formed on the optical shutter 84 is projected through the collimator lens 83 onto a front road surface.
The light distribution control circuit 88, which has received the detection signals of the oncoming vehicle detection sensor 86, the attitude change detection sensor 89, the speed sensor 90, and the steering rotation angle detection sensor 91, calculates the leveling of the basic axis of the desired beam, the beam distribution, and the beam intensity in accordance with the elevation angle, the traveling speed, and the traveling direction of the vehicle, and further the presence or absence of the oncoming vehicle.
The calculated result is input to the optical shutter drive circuit 87 as an output signal, and is converted by the optical shutter drive circuit 87 into a matrix signal for forming a desired optical image on the optical shutter 84 for output. In this manner, the optical shutter 84 forms an optical image for irradiating a predetermined light distribution beam onto the front road surface with reference to the position of the basic axis of the predetermined beam, and this optical image formed on the optical shutter 84 is projected to the front road surface by the collimator lens 83.
The color (white) of the light emitted from the lamp is determined by the light source adopted. However, it is known that the color of a light source varies among individual products depending on manufacturing conditions (manufacturing lot), and also varies within the individual products due to the ambient temperature, the lifetime, and the like. In particular, since a white light source using a semiconductor light-emitting element such as a light-emitting diode (LED) is a combination of a blue light-emitting element and a yellow fluorescent material, for example, not only the characteristic change of the light-emitting element but also the characteristic change of the fluorescent material affects in combination on the color, so that the difference or change of the color of the light source becomes remarkable.
When such a lamp is used alone, the color of the light emitted from the lamp using such a light source is not felt uneasy and is often unnoticeable even if it changes. However, when a plurality of lamps are used while they are arranged side by side, the difference in color of white light between the lamps is often noticeable. Further, when a road surface light distribution is generated by combining images by a plurality of separate lamps, there arises a problem that the color of white light becomes discontinuous at the boundary of the light distribution by each lamp in addition to the appearance. Such a difference in color of white light from a light source is often increased as the use is repeated even if the difference is small in the initial stage of the just-manufactured lamp.